The key biological processes leading to the formation of highly aggressive and drug-resistant epithelial ovarian cancer, which is the leading cause of death from gynecological malignancies, are not clearly understood. Second line chemotherapeutic agents have generally poor responses (~25%) that are associated with toxicity, and we have no biomarkers available how to sequence them for optimal efficacy. This emphasizes the need for both an improved understanding of disease resistance as well as effective treatment options to extend survival and enhance quality of life of the patients. The pegylated liposomal formulation of doxorubicin (DOX), known as (PLD), is one of the best tolerated second line of treatment, which exerts not only a cytotoxic antitumor effect but can also enhance antitumor immune responses. Recently, we have demonstrated a synergistic interaction between oncolytic vaccinia virus (OVV) and DOX or PLD in inducing immunogenic cell death in paclitaxel- and carboplatin-resistant ovarian tumors that was associated with curative inhibition of tumor growth in syngeneic and xenograft tumor models in mice (Mol. Therapy-Oncolytics, 2016). However, the mechanisms responsible for the synergistic interaction between OVV and DOX in killing drug-resistant ovarian tumor variants are unknown. Several lines of evidence indicate that DOX blocks proliferation of cancer cells through proteolytic activation of the membrane-bound transcription factor CREB3L1. The involvement of CREB3L1 during vaccinia-induced sensitization of tumor cells to DOX treatment elusive. Here, we propose to analyze expression of CREB3L1 in clinical specimens of ovarian tumor to explore its association with sensitivity to DOX treatment and the involvement of CREB3L1 in vaccinia virus-mediated increased responses to this drug. The specific aims are: i) We will analyze associations between CREB3L1 expression and sensitivity to DOX in clinical specimens from patients with serous ovarian cancer to explore a prognostic value of this marker in drug resistance, and ii) We will analyze the role of CREB3L1 in OVV-mediated susceptibility to DOX-induced cell death using human ovarian cell lines expressing CREB3L1 and their negative variants generated by genetic ablation. We anticipate that the insight obtained from understanding the cellular and molecular mechanisms associated with the synergistic interaction between OVV infection and DOX will benefit ovarian cancer patients, especially since oncolytic vaccinia viruses are being tested in clinical trials. Importantly, availability of a good biomarker is necessary to predict which patient responds to PLD, and also if oncolytic vaccinia viruses and PLD could be combined to enhance survival and quality of life.